Nanocomposites of polymethyl methacrylate (PMMA)/cellulose nanocrystals (CNC) have been developed through physical mixing of different ratios of CNC with dissolved PMMA. Solvent exchange method has been carried out in order to increase the dispersion of CNC in the polymer matrix. The obtained composite films showed high transparency. Moreover, its barrier properties against water vapor were significantly increased. The nanocomposite films were further coated with a layer of polyaniline (PANI) which showed a negligible electrical conductivity as confirmed by electrical properties investigations, but imparted antioxidant properties on the coated films. The inhibition ability reached up to 45% after 4 h and did not decrease upon storage for 3 months. Moreover, the coated nanocomposite films showed good antimicrobial activities against food poisoning microorganisms. The obtained results indicate that such modified PMMA composites could be a promising candidate for active food packaging.